And more over on the other trunk.
We’re forecast to have a night in the mid-20’s a week from now, and that takes me into one of my danger zones temperature-wise. For all of my temperate-zone trees, the cutoff point for staying on the bench is about 27F (with one exception – see below). Below that point, it’s time for my trees to start going on the ground. As I was mentioning this to Cathy over coffee, it occurred to me that I’ve never laid out my guidelines for protecting trees in a blog (at least not that I can recall). So here goes. Bear in mind that, first of all, the lowest temperature we’ve experienced in my current location is 15F (over a couple of days, with temps not getting above about 25). Normally in the course of the winter we range down to about 28 or so, a few times spaced weeks apart, with maybe 5-10 nights below freezing overall. The low temps only persist for four or five hours, and then we’re back above freezing. So our winter weather is truly mild (though I hate the cold and it’s hard for me to admit that).
A list of species and what I’ve experienced (your mileage may vary):
- Bald cypress – did fine on the bench at 15F frozen in blocks of ice for a couple of days; but go on the ground under 27F
- Crape myrtle – also did fine on the bench at 15F; go on the ground under 27F
- Beech – was killed at 15F (despite the fact that the species ranges all the way to Canada naturally); go on the ground under 27F
- Water-elms – all killed at 15F except for one large specimen in a large tub; this species is more cold-sensitive than my other temperate-zone trees, so they go on the ground under 30F
- American elm – did fine on the bench at 15F; go on the ground under 25F (this species also ranges to Canada)
- Winged elm – had at least one die at 15F; go on the ground under 27F
- Chinese elm – a very small specimen survived 15F on the bench frozen in ice (I was amazed); go on the ground under 27F
- Cedar elm – no data at 15F but they have done fine at 17F; but still go on the ground under 27F
- Hawthorns – survived 15F on the bench; go on the ground under 27F
- Sweetgum – survived on the bench at 15F (very surprising); go on the ground under 27F
- Roughleaf dogwood – survived on the bench at 15F frozen in ice; lost two on the bench in a later season at 22F thinking they should be all right; now go on the ground under 27F
- Oaks – mixed bag here, lost one or more Water oaks on the bench at 15F, my specimen Willow oak survived 15F frozen in ice; all go on the ground now under 27F
- Chinese privet – lost at least one on the bench at 15F (died back significantly); go on the ground below 27F
You probably noticed that my magic temperature number is 27F for most everything. As I said, our lows only persist (typically) for 4-5 hours and this cutoff has proven safe for me. If where you are you experience sub-freezing weather with no warmup for several days, you may need to adjust your practices accordingly.
It’s worth mentioning that whenever possible I will put my trees on the ground right under the bench in their individual spots. This provides not only the latent heat of the earth near the root zone, it also protects from radiant heat loss in the part of the tree above the soil. This is not always possible, of course, but generally speaking larger specimens have more cold resistance than smaller ones.
I also always make sure to point out that each of us has a mini-environment in our individual backyards. What works for one of us in terms of care – watering, amount of sunlight, winter cold protection – might not work for all of us. It never hurts to be cautious, even though it’s not fun moving a hundred trees around four or five times every winter. That’s part of the bonsai game, though, and if you accept that it is I can almost guarantee you fewer winter casualties.